1. Field of the Invention
The present disclosure relates to an X-ray tube equipped with a rotating anode cartridge comprising a reinforced sealing system. Embodiments of the claimed invention can be applied to special advantage but not exclusively in the field of X-ray tubes of an X-ray imaging system, such as an X-ray tomography or mammography system. Embodiments of the claimed invention may also be used in the field of non-destructive testing, when very powerful X-ray tubes are used.
2. Description of Related Art
In the field of radiology by X-rays, in particular, the X-rays are produced by an electronic tube equipped with an anode in rotation on a shaft. A powerful electric field created between the cathode and the anode enables the electrons emitted by the cathode to strike the anode, generating X-rays. For this emission, the positive polarity is applied to the anode via its shaft, the negative polarity to the cathode. The insulation of the assembly is assured, in particular, by dielectrics or by an enclosure, partially in glass, of the electronic tube.
When the tube is used at high power, the impact of electrons on the anode has the effect of abnormally heating up said anode. If the power is too high, an emitting track of the anode may be damaged, hollowed out with impact holes. To avoid such overheating, the anode can be rotated, so as to present, in front of the flow of electrons, a constantly renewed and always cold surface.
A motor of the tube therefore drives the shaft of the anode freely in a mechanical bearing. This bearing is situated in an anode chamber. The anode chamber is itself formed in a support of the anode. The bearing is maintained on the one hand by the anode support and maintains on the other hand the shaft of the anode.
In practice, the bearing industrially comprises conventional ball bearings, as opposed to rarely used magnetic bearings. The problem posed by rotating anodes stems from the rapid wear of the metal coating on the ball bearings, when the shaft rotates in the bearing. The lifetime is then around one hundred hours, leading to a period of use of the tube of around six months to a year. To overcome this problem, coating the ball bearings by metal, lead or silver in the form of a thin layer has been envisaged.
To reduce this premature wear of the metal layer, the invention also provides to place a lubricating film at the interface between the surfaces of the ball bearings and the shaft, between the bearing and the shaft of the anode. With this aim, a liquid based on gallium, indium and tin is poured inside the chamber. Such a liquid is chosen because it improves the coefficient of friction, it reduces the noise of impacts between the ball bearings and it increases the transfer of heat, due to the heating up of the anode, towards the fixed part, either by convection or by conduction. Other lubricating liquids are not used because they have poor degassing properties.
At present, the power demanded of electronic tubes is increasing with the aim of improving the diagnosis. This increase in power leads to an increase in the weight of the anode, up to six to eight kilograms. Consequently, the effects within the bearing become critical. Moreover, in a use in a tomodensitometer, continuously rotating at two rotations per second, the bearing undergoes an acceleration corresponding to around eight times the force of gravity g. Rotation speeds of three to four rotations per second are expected. Consequently, the lifetime of the bearing, and therefore of the tube, with ball bearings and the liquid, may be limited over time. Indeed, the liquid may lose its properties and therefore its characteristics as the heating and the friction within the bearing continue.
In addition, the use of a rotating anode must be compatible with three principal constraints. Firstly, the rotation of the anode must be as free and as perfect as possible, and simple dynamic balancing solutions must be provided to prevent the tube from vibrating when the anode rotates. Secondly, the anode must be able to be taken to a high electric voltage compared to the cathode (normally, bearings with steel ball bearings are used for this purpose). Thirdly, the heat produced by the impact of the electrons on the anode target and which propagates in the shaft must be evacuated efficiently.
Patent application FR-A-2 879 809 discloses an assembly in which ball bearings are lubricated by a gallium alloy and a sealing device of this assembly. In this assembly, an X-ray tube cartridge comprises an anode shaft fitted with ball bearings within a chamber of a fixed support. Such bearings are well suited to the considerable centrifugal accelerations undergone by the tube when it is fitted in a tomodensitometer.
The anode shaft is immersed in a liquid alloy in the chamber of the cartridge. The chamber is completely filled with this alloy. The document FR-A-2 879 809 provides that the sealing of the chamber is achieved by a sealing joint placed at the shaft outlet. An example of such a sealing joint is illustrated in FIGS. 1 and 2.
In FIGS. 1 and 2, the shaft 10 is maintained in the chamber by bearings. At the outlet 11 of the shaft 10, a receptacle or, in a general manner, an anchoring device, is provided to receive an anode 12. At the outlet 11, the fixed support of the chamber is fitted to a mounting ring 13.
The sealing of such a tube will be achieved in two complementary manners. Firstly, for the vacuum tightness, when the anode shaft 10 is not rotating, a space between an interior diameter of the ring 13 and an exterior diameter of the shaft 10 at the point directly in line with this ring 13 is limited. The limit of this space is fixed by the surface tension of the alloy of liquid gallium, indium, tin metal on the material of the shaft 10 and the ring 13. The ring 13 is intended to be fixed when the shaft 10 rotates.
When the shaft 10 rotates, the pressure of the liquid alloy increases. The alloy tends to escape from the chamber and to contaminate the enclosure of the tube. In this case, to confine it within the chamber, the invention provides to equip the surface of the ring 13, which is in contact or that of the shaft 10 directly in line with the ring 13, with a groove 14 of helix relief shape. The pitch of the helix is oriented so that, for a given direction of rotation of the shaft 10, the helix relief behaves like a scraper in front of the surface that rotates before it. Such a scraper tends to push the alloy back towards the chamber.
However, this type of sealing has disadvantages. Indeed, with this type of sealing joint, any small variation in the space between the interior diameter of the ring 13 and the exterior diameter of the shaft 10 leads to a loss of efficiency. Indeed, the increase in this space leads to a leak of the liquid alloy in the enclosure of the tube. A reduction in this space leads to friction.